Manufacture of propellant powder



Patented Feb. '27, 1945 MANUFACTURE OF PROPELLANT POWDER Bernhart Troxler, Kenvil, N. J., asslgnor to Hercules Powder Company, Wilmington, Del., a

corporation of Delaware No Drawing. Application April 11, 1941, Serial No. 388,084

3 Claims.

This invention relates to the manufacture of smokeless powder and more particularly to an improved method of removing solvents from smokeless powder grains.

It is well known that in the manufacture of smokeless powder nitrocellulose is colloided with a solvent or an admixture of solvents, such as ether, acetone and alcohol and then after the colloiding steps, the powder is pressed into strands that are of a diameter suitable for use as powder grains. These strands are cut into the proper length grains and the grains themselves treated to remove the excess solvent in order that the desired ballistics may be obtained. Practically all specifications for powders have upper limits as to the quantity of solvents which may be retained in the powder grains. This limit is usually quite low and requires that the grain be treated for'a sufficient time and in the proper manner to reduce the solvent content to desired percentages.

It is the object of this invention to provide a new and improved method of removing the solvents from the smokeless powder grains.

It is a further object of this invention to provide a method of removing solvents from smokeless powder grains which provides an added degree of safety to the operation.

It is a further object of this invention to remove the solvents from smokeless powder grains in a system that will not support combustion.

It is a still further object of this invention to provide a means of removing solvents from the smokeless powder grains that maintains the powder grains in an inert atmosphere during the removal of the solvents.

Other and further objects of this invention will be apparent hereinafter.

The objects of this invention in general have been accomplished by removing the solvent from the smokeless powder grains by circulating through the grains a gaseous medium which is characterized by an inability to support combustion. In particular, the object of the invention has been accomplished by passing an inert gas such as, for example, nitrogen, carbon dioxide, helium, flue gas, exhaust gas and the like, through the smokeless powder grains disposed in a container and recovering any solvents absorbed by the gaseous medium, for example, by passing the gaseous medium through a condenser, scrubber or absorber, and then, if desirable, circulating the inert gaseous medium back through the smokeless powder grains within the container.

It has been found that an inert gaseous medium for use in the recovering and removing the solvents from smokeless powder grains may be produced by adding various gases to air in sufficient quantity to reduce the oxygen content thereof to a value below that which will support combustion. But it has further been found that the use of pure nitrogen, carbon dioxide, helium or admixture thereof and other like inert gases or admixtures are adaptable to a process of recovering and removing the solvent. In addition to the aforementioned gases, various type flue gases and exhaust gases which have their oxygen content so diminished that they will not s port combustion may also be used as the inert gaseous medium. When exhaust or flue gases from internal combustion engines or combustion chambers are used, they preferably are scrubbed to remove any corrosive or other harmful ingredients.

The use of an inert gaseous medium for removing solvents from smokeless powder grains is applicable to the removal of solvents from grains of single base powders or from grains of double base powders. In the production of single base powders, various solvent such as, for example, ethyl alcohol, ethyl ether, acetone and the like have been used and must be recovered in order to operate economically and also must be removed in order to produce a ballistically satisfactory powder. In the production of double base powders, the solvent is present in minor amounts and sometimes is not worth recovering. However, it will be appreciated that removal and recovery of solvents from either single or double base powders is contemplated by this invention by the circulation of an inert atmosphere through the grains of single or double base powder.

In order to more particularly point out my invention, a brief description of the manufacture ,of smokeless powder is given with especial emphasis being placed on the solvent removal and recovery process and the detailsof the operation thereof given.

The powder is manufactured by colloiding it with a solvent or an admixture of solvents after which it is pressed into strands and passed through a cutter which granulates the strands to the desired size grains. The powder as it comes from the cutter thus has a definite granular form and contains a considerable amount of solvent which must be removed. The solvent which is contained in the grains in single base powder usually is an admixture of ether and a1- cohol. However, the single base powders may be manufactured with acetone or other solvents which are suitable for colloiding the nitrocellulose. The granular powder obtained from the cutter is subjected to a drying treatment for the purpose of removing the solvent. This drying treatment is of utmost importance in producing a uniform finished powder which will meet ballistic standards, and in addition, provides a means whereby a large portion of the solvent contained in the grains is recovered. The recovery of the solvent is incidental to the manufacture of the powder because only the removal from the powder is necessary to produce an acceptable propellant for ordnance work.

In general, there are three types of equipment that have been successfully used in removing solvents from and thereby accomplishing the initial drying of the smokeless powder. First, there is a chamber type in which the powder is placed in a metal lined box provided with a water sealed lid. Second, there is the car type which utilizes a solvent recovery car and a drying chamber, the car being loaded directly from the powder cutter and third, there is the tank type from which the powder is transferred from the cutter room and placed in a large metal tank. The tank type equipment and its operation arefully described in my copending applications Serial No. 325,515, filed March 23, 1940, and Serial No. 332,676, filed May 1, 1940. The operations in accordance with. the present invention are found to be most preferable when used in conjunction with the disclosures of the above mentioned copending applications.

The normal method of operating any one of the three described methods for removing solvents in the solvent recovery operation is to circulate warm air through the powder grains and then force the warm air which has picked up considerable solvents past a condenser comprising cold coils. The change in temperature effected by the coils causes a precipitation of the solvents upon the coils and produces in effect a miniature fog. The saturated air, after passing over these cold coils, loses its surplus solvent and may then'be reheated if necessary and again forced through the powder in a continuous cycle,

thus providing a completely closed system of air circulation. Instead of using cold coils or a condenser to remove the solvent from the air, absorbents or scrubbers may be employed, such as, for example, a charcoal filled tower or a packed column having a liquid scrubbing medium trickling over the packing. The length of the time of the treatment mainly depends upon the granulation of the powder, as larger caliber powders require a longer length of time than the smaller calibers. Also the composition of the powder effects the time of removal and the treatment may extend from a period of twelve hours to a week or longer. In order to determine when the solvent has been sufficiently removed from the powder grains, any suitable gauge or analyzing device may be employed. For example, a sight glass may be placed in the condensate line under the condenser and observation taken from time to time or a recording device placed in the system.

In the removal of the solvent from the powder grain, the process must be closely controlled since as the solvent is removed the grains partially collapse or shrink and close control is necessary to produce a uniform finished product. Furthermore, if the solvent is removed too rapidly, the surface of the powder grains becomes hard and it is much more diflicult, if not impossible, to

remove the solvent from the interior of the grain. Most operations have definite lengths of time for treating the powder grains in the solvent recovery system.

The solvent is removed from the powder grains to a fairly low percentage but as the concentration of the solvent in the grains falls, the removal slows down and it is impractical from an economic standpoint to reduce the solvent content of the powder to the desired degree in the solvent recoivery treatment. Hence, after the solvent recovery treatment an additional drying step is resorted to which may take the form of soaking the grains in warm water and then air drying the grains after the soaking, or thegrains may be taken directly from the solvent removal system and be air dried. After drying, the grains are screened, glazed if necessary, and packed ready for use.

In the foregoing discussion, removal of the solvent has been accomplished by means of air circulation, and this is at present the accepted practice in smokeless powder operations. However, it has been found that in some instances there is produced a solvent concentration which is subject to ignition, combustion or explosion in such a system. In order to prevent such a solvent admixture from being produced, an inert gaseous medium has been used as the gaseous medium in the recovery system. In accordance with this invention, the solvent removal and recovery step is accomplished by circulating an inert gaseous medium through the powder grains, removing from this inert gaseous medium the solvents which it takes up and recirculating the inert gaseous medium back through the grains. It has been found that by reducing the concentration of oxygen it is possible to provide a medium which will not support combustion and that by the use of this oxygen deficient medium which is characterized by an inability to support combustion, a much safer process of solvent removal and recovery is effected. The efficiency of the process is not adversely affected as the inert gaseous medium will carry substantially the same amount of solvent as the air medium which has been used heretofore.

While I have found it possible to use pure nitrogen, helium, carbon dioxide, and the like, I prefer to use scrubbed flue or exhaust gases, as they are the most economical mediums available.

It will be understood that in accordance with this invention the use of any inert gaseous medium is contemplated. Thus, if desirable, air may be enriched in nitrogen content by the addition of pure nitrogen to a point where the oxygen content thereof is insufficient to support combustion. Similarly, any oxygen containing gas may be used provided that the oxygen content thereof is reduced to a point where the gas has a concentration and composition which is characterized by an inability to support combustion.

In respect to the preferred types of inert gases, those with high heat capacity are generally more effective; for example, when air-ether vapor mixtures are diluted with carbon dioxide, no combustion will occur at an oxygen concentration of 8% or lower. When the diluent was nitrogen, this limit was reduced to 6.5% oxygen. The means whereby the ignitibility of the vapor mixtures was determined comprised contacting an alcohol vflame with the open bottom end of a asv aoa excess solventfrom saidgrains which consist in,

vertical tube containing the mixture under test. This procedure gave a more severe test than the use of a spark gap since the alcohol flame gave -a more positive ignition. Failure to ignite by this means insures the non-ignitibility of the vapors. It is apparent from the above results that both the carbon dioxide and nitrogen are efiective at suppression of explosibility of ether vapor and other solvent vapors.

The inert gases which are suitable for such applications are those which fail to support combustion by themselves and which do not exhibit undesirable effects on the smokeless powder colloid or on the apparatus. which are themselves combustible may be utilized, for example, the lower molecular weight hydrocarbon gases, difiicultiesmight be experienced by absorption of these vapors in the solvent recovery system or in the solvent wet powders, Also the occurrence of leaks in the apparatus might lead to a hazardous condition.

Although diluents It i preferable, therefore, to utilize inert-gases which are not themselves combustible when mixed with oxygen.

The advantages of using inert gaseous mediums in the solvent recovery system of smokeless powder manufacture will be apparent and since the efliciency, economy and safety of the operations are improved, the present process is highly desirable in the manufacturing procedure.

It will be understood that the details and examples given hereinbefore are illustrative only and in no way limiting on my invention as broadly described hereinbefore and in the apand cutting the strands into grains of any desired size, the steps of removing the bulk of the smokeless powder which includes colloiding a mixture comprising nitrocellulose with a solvent,

pressing the colloided nitrocellulose into strands and cutting the strands into grains of any desired size, the steps or removing the bulk of the excess solvent from said grains which consist in passin heated scrubbed flue gas through a mass of the said grains, removing the solvent from the said flue gas after its contacting with the'said grains by cooling the flue gas containing solvent suificiently to effect precipitation of the solvent, reheating the said flue gas after said'precipitation and removal of the solvent and recirculating the said reheated flue gas through the mass of smokeless powder grains.

3. In the method of manufacturing colloided smokeless powder which includes colloiding a mixture comprising nitrocellulose with a solvent, pressing the colloided nitrocellulose into strands and cutting, the strands into grains of any desired'size, the steps of removing the bulk of the excess solvent from said grains which.

consist in passing heated scrubbed exhaust gas through a mass of the said grains, removing the solvent from the said exhaust-gas after its contacting with the said grains by cooling the exhaust gas containing solvent sufficiently to effect precipitation of the solvent, reheating the said exhaust gas after said precipitation and removal of the solvent and recirculating thesaid reheated exhaust gas through the mass of smoke-' les powder-grains.

- BERNHART TROXLER. 

